This invention relates generally to prosthetic devices, and more particularly, to a prosthetic hip joint assembly.
Each hip joint in the human body includes an acetabulum or natural socket in the pelvis which receives the femoral head or ball, which is joined to the femur or thigh bone by a neck portion. As a result, a ball-and-socket joint is provided. In many instances, however, total hip joint replacement may be required. For example, due to various progressive deteriorative diseases, such as osteoarthritis, or abnormal stresses applied to the hip joint, the hip joint, and particularly the femur neck, may fail. In some cases, only the femoral head and neck require replacement, while in other cases, the acetabulum must also be replaced.
Various hip joint prostheses have been proposed. Generally, an acetabular cup is cemented or press fit into the acetabulum socket in the bone. A femoral member comprising an elongated stem, a neck and a head are mounted to the femur by cementing or press fitting the stem into the intramedullary canal of the femur. The femoral head is then received in a bearing insert within the acetabular cup to provide a ball-and-socket movement therein.
In some instances, a bearing insert is provided within the acetabular cup which receives the femoral head, and the acetabular cup articulates within the acetabulum. For example, in U.S. Pat. No. 3,813,699 to Giliberty, a prosthetic hip joint is proposed having an acetabular cup, a bearing insert received in the acetabular cup and a femoral head received in the bearing insert for ball-and-socket movement therein. The bearing insert has a lower open end with a narrowed diameter which defines the entranceway into the interior of the bearing insert. In order to assemble the femoral head in the bearing insert, the femoral head is pressed into the bearing insert such that the reduced diameter open end thereof yields slightly under pressure. After the largest diameter portion of the femoral head passes the reduced diameter opening of the bearing insert, the reduced diameter opening regains its initial shape so as to retain the femoral head therein. However, with such assembly, the femoral head may dislocate and escape from the bearing insert when the leg or body is abnormally twisted. This, of course, requires a new surgical procedure to reinsert the same. Orthopedic prosthetic implant devices of a similar nature are described in U.S. Pat. No. 3,863,273 to Averill; U.S. Pat. No. 4,044,403 to D'Errico; and U.S. Pat. No. 4,408,360 to Keller.
Various other hip prosthetic devices which employ an articulating type of acetabular cup, a bearing insert mounted in the acetabular cup and a femoral head further include a retaining ring secured to the acetabular cup in order to retain the femoral head and bearing insert therein. Examples of such devices are disclosed in U.S. Pat. Nos. 3,818,512 to Shersher; 3,889,299 to Osborne et al.; 4,380,090 to Ramos; and 4,770,661 to Oh. However, with these devices, the retaining ring is inserted after the femoral head is positioned within the bearing insert in the acetabular cup. It will be appreciated, however, that because of the extremely small space at the hip joint, it is difficult to handle such retaining rings. For example, U.S. Pat. No. 3,889,299 to Osborne et al. requires screws to connect the different parts of the retaining ring together. In such case, it would be extremely difficult to insert a screwdriver in such a small space. In U.S. Pat. No. 3,818,512 to Shersher, the retaining ring must be threadedly received within the acetabular cup, and accordingly, a tool must positioned within the hip joint in order to turn the retaining ring a plurality of times in order to secure the same in the acetabular cup, which is extremely difficult in the small space. In U.S. Pat. No. 4,380,090 to Ramos, a thin locking ring must be positioned within a narrow groove in the acetabular cup, which again is extremely difficult to manipulate with such a thin locking ring. Finally, in U.S. Pat. No. 4,770,661 to Oh, the retaining ring is comprised of two separate parts, and therefore additional space must be provided for securing the two parts together at the hip joint.
In U.S. Pat. No. 4,241,463 to Khovaylo, there is disclosed a prosthetic implant device for a hip joint. In this case, the bearing insert is provided with an upwardly and outwardly inclined inner, annular wall. A split ring is positioned within the recess provided thereby so that, as the femoral head is inserted into the bearing insert, the split ring moves up the inclined wall and moves apart to permit the femoral head to enter the inner dome of the bearing insert, whereupon the split ring falls back down. If a pulling force is applied to the femoral head to remove the same, for example, during an abnormal twisting or torquing movement, a wedging effect takes place with respect to the split ring to prevent removal of the femoral head. In actual practice, however, such an assembly has proven unsatisfactory since the wedging effect does not completely prevent removal of the femoral head. As a result, in the actual commercial models of the Khovaylo device, the bearing insert has been provided with an inner, annular ledge which more positively prevents the split ring from escaping, even when the pulling force on the femoral head is great. However, with Khovaylo, because of the large space required for sliding of the split ring during insertion of the femoral head, the dimensions of the split locking ring must be made smaller. Accordingly, there is less bearing contact of the split ring with the femoral head when the latter is inserted within the bearing insert, and less contact area at the opposite surface of the split ring with the wedging, locking surface, thereby permitting easier escape of the femoral head from the bearing insert. In addition, because of this movement, and the strength requirements that are necessary on the smaller split ring, there is a restriction on the types of materials that can be used to construct the same. Of a similar nature is U.S. Pat. No. 3,862,807 to Doden, which has even less contact of the split ring with the ball shaped member, that is, in which there is only an effective line contact when the femoral head is inserted within the bearing insert. This is due to the rectangular cross-section of the bearing insert.
U.S. Pat. No. 3,787,128 to Maistrelli discloses a device in which a split retaining ring is inserted after the ball is inserted within the socket, and would therefore suffer from the same problems as Khovaylo.
U.S. Pat. No. 3,683,421 to Martinie discloses a prosthetic joint assembly which is much less relevant than the above references, but which is included herein for a complete review of the prior art.